Previous to TestFlight, life was ridiculously painful for an iPhone developer to beta test apps. I wish they'd charge me, it's that good. I'd probably pay at least $100 a month.
My guess is an eventual business model is for advertisers (the true customers) to get user information from developers who opt-in. Getting a mass-load of app developers, each with their own end users, into your ecosystem provides a lot of data to sell.